This invention relates to a semiconductor light emitting device and a semiconductor light emitting apparatus using a transparent substrate, and more particularly to a semiconductor light emitting device and a semiconductor light emitting apparatus having a structure suitable to achieving sufficient optical output.
Semiconductor light emitting devices, especially light emitting diodes (LEDs), have been widely used for such applications as full-color displays, traffic signal equipment, and in-vehicle applications. These applications particularly require the devices with higher optical output.
Conventionally, typical LEDs of this type have a structure in which a light emitting layer having a p-n junction is formed on the upper surface of a transparent substrate having a generally rectangular cross section and being translucent to the emission wavelength. For the purpose of electrical connection, the light emitting layer is provided with an upper surface electrode on its upper surface side and a lower surface electrode on its lower surface side.
In an LED configured as described above, part of the light emitted from the p-n junction has an incident angle not greater than the critical angle and can be extracted outside the LED. However, light having an incident angle greater than the critical angle is totally reflected, subjected to multiple reflections inside the LED, and finally vanishes by absorption in the light emitting layer or the transparent substrate. This presents a problem that LEDs having a high optical output cannot be obtained. In this respect, an LED having an upper surface on which a roughened light extraction region is formed, and an LED having a lower surface on which a recessed portion of generally spherical shape is formed, are known (see, e.g., Japanese Laid-Open Patent Application 10-200156 (1998), page 3, FIG. 4 (hereinafter referred to as Patent Document 1); or Japanese Laid-Open Patent Application 9-92878 (1997), page 4, FIG. 1(hereinafter referred to as Patent Document 2)).
The LED disclosed in Patent Document 1 is an LED of the so-called mesa- structure, which has downward curved surfaces in the upper portion of an AlGaAs semiconductor substrate having a p-n junction. The LED comprises a light extraction region made of a rough surface on the upper surface of the p-type semiconductor region, a light reflection region having a collection of numerous microsurfaces on the lower surface of the n-type semiconductor region, and another light extraction region made of a rough surface on the curved surface formed by mesa etching.
The LED disclosed in Patent Document 2 has a light reflection region with a recessed portion of generally spherical shape formed on the lower surface of an AlGaAs semiconductor substrate having a p-n junction by photolithography and etching.
However, the LED disclosed in Patent Document 1 or 2 described above has a problem that, when the LED has a chip size as large as, for example, 0.5 to 1 mm in order to obtain higher optical output, light emitted from the light emitting layer is subjected to multiple reflections inside the LED and absorbed by the light emitting layer many times, which increases the proportion of vanished light and prevents accomplishment of a LED with higher optical output.